1. Field of the Invention
The present invention relates to a mobile communication terminal apparatus used in a cellular radio communication system.
2. Description of the Related Art
Recently, cellular radio communication systems have become popularized. In a cellular radio communication system, a plurality of base stations are distributed in a service area, and radio zones called cells are formed by these base stations. In each of these cells, radio connection is established between the base station and each mobile communication terminal apparatus. In this type of system, when a mobile communication terminal apparatus is powered on, synchronization is established between the mobile communication terminal apparatus and the nearest base station. After the establishment of synchronization, the apparatus transitions to a standby state.
In a standby state, the mobile communication terminal apparatus performs intermittent reception operation. In the intermittent reception operation, a wakeup period and sleep period are alternately set in a predetermined standby cycle so as to reduce the power consumption of the mobile communication terminal apparatus. The standby cycle is defined by the system.
In a wakeup period, the mobile communication terminal apparatus sequentially performs operation for wakeup, reception timing identification operation, and paging reception operation. According to W-CDMA (Wideband Code Division Multiple Access) (IMT-2000: 3GPP standards), for example, operation for wakeup, reception timing identification operation, and reception operation for a paging indicator channel (PICH) are sequentially performed.
Of these operations, in reception timing identification operation, first of all, the time average of the reception levels of radio signals arrived from a base station during synchronization establishment (to be referred to as an active base station hereinafter) is detected in a preset reception level measurement interval. The time average of reception levels is detected to suppress the influences of interference and noise. A path as a reception target is then selected on the basis of the detected time average of reception levels, and reception timing identification is performed for the selected path. In PICH reception operation, PICH reception is performed in accordance with the identified reception timing. When an incoming message addressed to the home terminal is detected, reception operation for termination control is performed in succession. If no incoming message addressed to the home terminal is detected, the apparatus performs operation for transition to a sleep period.
Mobile communication terminal apparatuses have recently been required to have longer continuous standby times. For this reason, various schemes of further reducing power consumption in intermittent reception operation have been proposed.
In the first scheme, for example, only when the reception level of a radio signal from an active base station is less than a threshold, a base station for which synchronization should be established is switched from the active base station to a neighboring base station. This switching processing is generally called reselection.
After the above PICH reception, the mobile communication terminal apparatus compares the detected value of the reception level of a radio signal from the active base station with a threshold. If the detected value is equal to or more than the threshold, the apparatus directly transitions to a sleep period. In contrast to this, if the detected value is less than the threshold, the apparatus searches for a neighboring base station without transitioning to a sleep period. The apparatus then compares the detected value of the reception level of a radio signal from the neighboring base station detected by this search with the detected value of the reception level of the radio signal from the above active base station. If a neighboring base station is found, which is higher in the detected value of reception level than the active base station by a predetermined level or more, reselection is executed. If no neighboring base station is found, which satisfies the above condition, the apparatus transitions to a sleep period.
According to the second scheme, a radio telephone apparatus is designed to measure the reception levels of radio waves from a base station in communication and other base stations for zone transition determination. In this scheme, the apparatus obtains the differences between the reception level of a radio wave from a base station in communication and the reception level of a radio wave from each of other base stations. As this difference increases, the frequency of measurement of the reception levels of radio waves from neighboring base stations is decreased. As the difference decreases, the frequency of measurement of the reception levels of radio waves from neighboring base stations is increased. This scheme is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-285911.
The use of these schemes will omit the operation of measuring the reception levels of radio signals from neighboring base-stations or reduce the frequency of measurement while the reception levels of radio signals from an active base station are high. This makes it possible to reduce the power consumption of the terminal apparatus in a standby state as compared with a case wherein the reception levels of radio signals from neighboring base stations are always measured regardless of the reception levels of radio signals from an active base station.
In each of the first and second schemes, however, the reception level measurement interval is always fixed in both cases wherein the reception levels of radio signals from an active base station are to be measured and the reception levels of radio signals from neighboring base stations are to be measured. For this reason, every time a reception level is measured, a predetermined amount of power is consumed.
In order to reduce the power consumption due to this reception level measurement, the reception level measurement interval may be shortened. If, however, the reception level measurement time is simply shortened, the interference and noise suppression effects become insufficient. In addition, a trouble occurs in reception timing identification, and the PICH reception performance deteriorates. Furthermore, the measurement precision on the reception levels of radio signals from an active base station and neighboring base stations deteriorates, and variations in measured values increase. For this reason, even reselection that need not be done as long as measurement is executed with high precision is done. As a consequence, the wakeup period increases, resulting in an increase in power consumption.